Aluminum silicon alloys

ABSTRACT

A method of preparing an improved hypereutectic aluminum-silicon alloy is disclosed, including the resulting composition. One mode comprises the addition of sintered aluminum powdered rods containing aluminum oxide particles, uniformly distributed throughout, to a hypereutectic melt of aluminum-silicon. It further comprises the addition of sodium in low concentrations added from a master Al-Si-Na alloy to the hypereutectic melt of aluminum-silicon. The melt is solidified producing a composition having both primary and eutectic silicon simultaneously finely divided and uniformly distributed throughout the casting.

United States Patent 1 Bolling et al.

[451 July 22,1975

1 1 ALUMINUM SILICON ALLOYS [75} Inventors: Gustaf Frederic Bolling,Dearborn,

Mich; Jean Ciss, St. Germain en Laye, France [73] Assignee: Ford MotorCompany, Dearborn,

Mich.

[22] Filed: Oct. 1, 1973 [21] Appl. No.: 402,527

[52] US. Cl. 75/148; 75/68 R; 75/141; 75/142; 148/32; 148/325 [51] Int.Cl. C22C 21/04 [58] Field of Search 75/148, 141, 142, 143, 75/68 R, 138,146, 147; 148/32, 32.5

[56] References Cited UNITED STATES PATENTS 3,705,029 12/1972Foerster.... 75/148 Primary ExaminerR. Dean Attorney, Agent, orFirmJoseph W. Malleck; Keith L. Zerschling [57] ABSTRACT A method ofpreparing an improved hypereutectic aluminum-silicon alloy is disclosed,including the resulting composition. One mode comprises the addition ofsintered aluminum powdered rods containing aluminum oxide particles,uniformly distributed throughout, to a hypereutectic melt ofaluminum-silicon. It further comprises the addition of sodium in lowconcentrations added from a master Al-Si-Na alloy to the hypereutecticmelt of aluminum-silicon. The melt is solidified producing a compositionhaving both primary and eutectic silicon simultaneously finely dividedand uniformly distributed throughout the casting.

8 Claims, 4 Drawing Figures ALUMINUM SILICON ALLOYS BACKGROUND OF THEINVENTION Hypereutectic aluminum-silicon alloys have been deemedsignificant for use by the casting industry as well as the automotiveindustry. This principally results from the potential that such alloyshold for providing good wear resistance along with the conventionaladvantages derived from aluminum castings. Furthermore, it is wellrecognized that the casting characteristics of hypereutectic alloys arevery good.

It is known that increasing the amount of silicon in aluminum-siliconalloys will increase wear resistance. The existence of the primarysilicon phase has been considered the principal reason for the increasein wear resistance. Only limited or insignificant consideration has beengiven by the prior. art to the role played by eutectic silicon withrespect to the wear characteristic.

The prior art has further appreciated that small and well dispersedparticles of primary silicon in an aluminum-silicon eutectic matrix willimprove wear resistance and other physical characteristics. To this end,commercial refiners or modifiers have been developed to effect eitherrefinement of primary or eutectic silicon, but not refinement of both ina single casting. Phosphorous is one of the commercial refiners that hasbeen used to achieve finely dispersed particles of primary silicon. Aproblem associated with the use of phosphorous is pollution resultingfrom phosphorous salts. Sodium is one of the commercial refiners thathas been used to achieve refined eutectic silicon. But the refinementresults from a lower growth temperature which changes the mode ofgrowth, rather than the refinement resulting from an increase in thenucleation rate of the eutectic silicon. A problem associated with thesimultaneous use of phosphorous and sodium is the formation of sodiumphosphides which effectively inhibit the separate chemical or nucleatingaction of phosphorous or especially sodium insofar as it is a refiner ofsilicon in aluminum-silicon alloys. This may in part be explained by thefact that sodium will normally be added above the liquidus temperatureand hypereutectic silicon requires an increased melting temperature;thus increased sodium losses occur due to its vapor pressure.

SUMMARY OF THE INVENTION One of the primary objects of this invention isto provide a hypereutectic aluminum-silicon alloy which has improvedwear resistance and machineability and mechanical properties.

Another significant object of this invention is to provide an improvedmethod for refining primary and eutectic silicon in an aluminum-siliconmelt while overcoming the problems associated with prior-art refiners. Aparticular feature of the method is the introduction of aluminum oxideto the melt (in one preferred mode, the aluminum oxide is introduced ina finely divided and uniformly dispersed condition) for therebyproviding nucleating sites for the primary silicon. A further andconnected feature of the method is the provision for the refinement ofthe eutectic silicon made possible by the addition of sodium along withaluminum oxide; it has been discovered that sodium is no way renderedinert by reaction with the aluminum oxide inoculant. Simultaneousrefinement of both the primary and the eutectic silicons is therebyprovided.

DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 show microstructures for analuminumsilicon hypereutectic alloy under various magnifications, themicrostructure depicting the inventive distinguishing characteristics ofthe novel composition herein and as a result of the novel method ofpreparation.

FIG. 4 illustrates the microstructure of a similar aluminum-siliconalloy microstructure which results from the addition of sodiuminfluencing the eutectic silicon while the primary silicon is in arefined state and which is a significant result of the novel use ofaluminum oxide as an addition to the hypereutectic melt.

ETAILED DESCRIPTION Evaluation of many commercial and experimentalaluminum alloys indicates that only the hypereutectic aluminum siliconalloy system contains potential for promoting high wear resistance ofthis system is attributed to the extremely hard primary, the influenceand presence of both silicon phases not being appreciated nor obtainedby the prior art simultaneously. Generally, as the silicon content ofthese alloys is increased. wear resistance accordingly increases. Largeamounts of silicon, however, can cause problems with machineability andcastability. Therefore, it is important that a sufficient amount ofprimary silicon be used to impart the required improvement in wearresistance, but not so much as to cause casting and machiningdifficulties. However, the physical and metallurgical improvementsobtained by this invention will be rendered in the broad range ofsilicon contents as long as the alloy is hypereutectic. To obtainoptimum performance, a narrower range of silicon may be desirable, aswill be described.

It has been discovered in accordance with this invention that ifaluminum oxide is introduced to the hot melt during the formulation ofthe aluminum-silicon alloy, the aluminum oxide will function as anucleant for primary silicon insuring that the majority of the siliconphases will be uniformly distributed and of a fine nature. For the firsttime, both phases (primary and eutectic) of the silicon can besimultaneously refined and modified to achieve unprecedented wearresistance and machineability by concomittent, precedent or subsequentmelt additions.

Optimum machineability and good performance of the hypereutectic siliconalloy requires a reasonably small size and uniform distribution of thesilicon phases. Two factors which govern the silicon particle size inthe phases are (1) solidification rate and (2) artificial nucleation.

For example, in a 390 silicon aluminum alloy containing l618% silicon,45% copper, 0.1% maximum manganese, O.6-l.l% iron, OAS-0.65% magnesium,0.1% maximum Zinc, 0.2% maximum titanium, traces of phosphorous and theremainder aluminum, the primary silicon crystal formation will begin atapproximately 1200F (liquidus temperature) and will be complete with theaccompanying start of the first eutectic solidification at about 1050F.Without a prior refinement treatment, silicon particle size iscontrolled principally by the rate of cooling through this temperaturerange; more rapid rates will result in finer particles. Even in refinedmelts, where the size is also influenced by an increased number ofnucleation sites, the ultimate primary silicon size will depend on thesolidification rate. A die-cast process is preferred because it willprovide an extremely rapid solidification rate and assist in producing avery fine particle size in conjunction with the use of aluminum oxide ofthis invention.

METHOD A preferred method in conformity with this invention comprises(a) preparation of a hypereutectic aluminum-silicon melt having apouring temperature of at least 1200F.; (b) introducing A1 0, to saidmelt in an amount such that the sum of silicon and A1 0 is about from19-21% by volume of the melt; (c) introducing a quantity of sodium inthe range of (LS-1.5% by weight of the melt; and (d) pouring the meltmixture into a diecast machine.

Ideally, to avoid formation and segregation of a solid phase during flowand to solidify entirely at a rate influenced by conditions in a moldcavity, a hypereutectic aluminum silicon alloy should fill the cavitybefore cooling below its liquidus temperature. This requires that heatlosses from the molten metal to the ladle, receiving chamber, runnersystem and during flow through the mold cavity itself, be considered inestablishing the correct holding furnace temperature. If the pouringtemperature of the melt is to be lowered, it is important that the totalvolume content or sum of the silicon and aluminum oxide in the melt bemaintained constant, in the range of 19-21% of the melt, while thealuminum oxide content is increased.

In order for the aluminum oxide to be optimally effective, it must bepresent in the melt as surface active. well dispersed. very fineparticles. However, nucleation of silicon on massive Al O particles ispossible even though nucleation would seemingly be impossible on anapparently inert surface. These conditions are facilitated by the way inwhich the aluminum oxide is introduced into the melt. It has beendiscovered that by the use of sintered aluminum powder rods containingaluminum oxide (for example about 10% relative to the content of themaster alloy) one effective mechanism results. The powdered rods areintroduced to the melt containing aluminum and hypereutectic content ofsilicon (for example such that in the dissolved nature, the aluminumoxide will constitute about 3% of the melt). This has been found to be avery effective way for adding aluminum oxide particles less than 10microns in size so that they will be immediately wetted or otherwiseinfluenced to act as wetted by the melt liquid. The silicon is thencapable of interacting immediately upon cooling below the alloy liquiduswith the aluminum oxide in order to provide active surfaces where theaforementioned primary silicon nucleation takes place with little or nonucleation of the aluminum.

To achieve an object of reducing pollution problems, aluminum oxide canbe used as a substitute for phosphorous which is conventionally used asa refiner in such types of hypereutectic aluminum alloys. Thus, theinvention eliminates pollution problems arising when phosphorous saltsare used.

COMPOSITION Referring now to the microstructural figures, andparticularly FIG. 1, there is illustrated a metallographic illustrationof a polished specimen, l50 magnification. Silicon represents 17% of thecomposition and aluminum oxide 3%. Those portions labeled a representprimary silicon nucleated from a cloud of aluminum oxide. Those portionslabeled [2 represent coarse eutectic silicon. Those portions labeled 1'represent a cloud of aluminum oxide within the matrix.

In FIG. 2, the same composition was analyzed by scanning microscopy, thespecimen being etched and photographed at 500 magnification. The primarysilicon nucleated from a cloud of aluminum oxide as shown in detail,primary silicon being represented by a and a cloud of aluminum oxidebeing labeled 0 acting as a nucleant. In FIG. 3, again the samecomposition is analyzed at [00X magnification for purposes of comparingwith FIG. 4 at a similar magnification, but incorporating sodium as anadditive. In FIG. 3, the same primary silicon and coarse eutecticsilicon is labeled a and b respectively; in FIG. 4 the interconnectedprimary silicon and aluminum oxide particles are labeled a and very fineeutectic silicon is labeled 1) (note the more globular shape of theprimary silicon).

The preferred range of 16-18% silicon coincides generally with the rangeof greatest fluidity of the aluminum silicon system at normal castingtemperatures, but an operational range is 16-19%. This is an importantconsideration for an alloy that is to be cast into complex shapes. Thealloys represented in FIGS. l-3 can be used in the as-cast orartificially aged and stabilized temper.

We claim:

1. In a method of producing aluminum-silicon castings by preparation ofa hypereutectic aluminumsilicon melt having a pouring temperature of atleast I200F and pouring the melt into a casting machine, the improvementconsisting of introducing aluminum oxide and sodium to saidhypereutectic aluminumsilicon melt to nucleate both a primary and aeutectic silicon phase in said melt upon cooling.

2. In a method of producing aluminum-silicon castings by preparation ofa hypereutectic aluminumsilicon melt having a pouring temperature of atleast 1200F and pouring the melt into a casting machine, the improvementcomprising:

a. introducing aluminum oxide to (a) said hypereutectic aluminum-siliconmelt so that throughout the melt, the aluminum oxide is surface activeor otherwise acts as so influenced, uniformly dispersed and has aparticle size no greater than 10 microns.

3. A method of making an improved hypereutectic aluminum-silicon alloy,comprising:

a. introducing a quantity of sintered aluminum powder rods having about10% aluminum oxide particles uniformly distributed throughout said rods,to a hypereutectic melt of aluminum and silicon so that the aluminumoxide constitutes about 3% of said melt. and

b. allowing said melt to solidify with clouds of aluminum oxideparticles acting as nucleating sites for primary silicon, the aluminumalloy having improved wear resistance and increased machineability.

4. A method as in claim 2, in which the volume content of silicon plusaluminum oxide in said melt is maintained approximately constant, whileincreasing the aluminum oxide to permit lowering the pouring temperatureof said melt.

5. A method as in claim 3, in which the aluminum oxide particles have asize no greater than 10 microns.

6. A method as in claim 3, in which sodium is added to said melt as arefiner for the eutectic silicon and inci- 6 and finely divided.

8. A hypereutectic aluminum-silicon alloy composi tion consistingessentially of l6l 9% silicon, 3% aluminum oxide. sodium in an effectiveamount less than 1.5% to refine the eutectic silicon, and the remainderaluminum, the microstructure of said composition being characterized byfine primary silicon being nucleated from a cloud of aluminum oxide.

1. In a method of producing aluminum-silicon castings by preparation ofa hypereutectic aluminum-silicon melt having a pouring temperature of atleast 1200*F and pouring the melt into a casting machine, theimprovement consisting of introducing aluminum oxide and sodium to saidhypereutectic aluminum-silicon melt to nucleate both a primary and aeutectic silicon phase in said melt upon cooling.
 2. In a method ofproducing aluminum-silicon castings by preparation of a hypereutecticaluminum-silicon melt having a pouring temperature of at least 1200*Fand pouring the melt into a casting machine, the improvement comprising:a. introducing aluminum oxide to (a) said hypereutectic aluminum-siliconmelt so that throughout the melt, the aluminum oxide is surface activeor otherwise acts as so influenced, uniformly dispersed and has aparticle size no greater than 10 microns.
 3. A method of making animproved hypereutectic aluminum-silicon alloy, comprising: a.introducing a quantity of sintered aluminum powder rods having about 10%aluminum oxide particles uniformly distributed throughout said rods, toa hypereutectic melt of aluminum and silicon so that the aluminum oxideconstitutes about 3% of said melt, and b. allowing said melt to solidifywith clouds of aluminum oxide particles acting as nucleating sites forprimary silicon, the aluminum alloy having improved wear resistance andincreased machineability.
 4. A method as in claim 2, in which the volumecontent of silicon plus aluminum oxide in said melt is maintainedapproximately constant, while increasing the aluminum oxide to permitlowering the pouring temperature of said melt.
 5. A method as in claim3, in which the aluminum oxide particles have a size no greater than 10microns.
 6. A method as in claim 3, in which sodium is added to saidmelt as a refiner for the eutectic silicon and incidentally as abeneficial shape modifier for the primary silicon.
 7. A HYPEREUTECTICALUMINUM-SILICON ALLOY COMPOSITION CONSISTING ESSENTIALLY OF 16-18%SILICON, AL2O3 IN AN AMOUNT SUCH THAT THE SUM OF SILICON PLUS AL2O3 ISGENERALLY ABOUT 20% BY VOLUME OF THE ALLOY, SODIUM BEING PRESENT IN ANEFFECTIVE AMOUNT LESS THAN 1.5% TO REFINE THE EUTECTIC SILICON,REMAINDER ALUMINUM, SAID COMPOSITION BEING CHARACTERIZED BY BOTH THEEUTECTIC SILICON AND PRIMARY SILICON BEING UNIFORMLY DISTRIBUTED ANDFINELY DIVIDED.
 8. A hypereutectic aluminum-silicon alloy compositionconsisting essentially of 16-19% silicon, 3% aluminum oxide, sodium inan effective amount less than 1.5% to refine the eutectic silicon, andthe remainder aluminum, the microstructure of said composition beingcharacterized by fine primary silicon being nucleated from a cloud ofaluminum oxide.